Discrete Mathematics 306 (2006) 3166–3173 www.elsevier.com/locate/disc Results on the Grundy chromatic number of graphs Manouchehr Zaker Institute for Advanced Studies in Basic Sciences, 45195-1159 Zanjan, Iran Received 7 November 2003; received in revised form 22 June 2005; accepted 22 June 2005 Available online 7 August 2006 Abstract Given a graph G,byaGrundy k-coloring of G we mean any proper k-vertex coloring of G such that for each two colors i and j, i<j, every vertex of G colored by j has a neighbor with color i. The maximum k for which there exists a Grundy k-coloring is denoted by (G) and called Grundy (chromatic) number of G. We first discuss the fixed-parameter complexity of determining (G)k, for any fixed integer k and show that it is a polynomial time problem. But in general, Grundy number is an NP-complete problem. We show that it is NP-complete even for the complement of bipartite graphs and describe the Grundy number of these graphs in terms of the minimum edge dominating number of their complements. Next we obtain some additive Nordhaus–Gaddum-type inequalities concerning (G) and (Gc), for a few family of graphs. We introduce well-colored graphs, which are graphs G for which applying every greedy coloring results in a coloring of G with (G) colors. Equivalently G is well colored if (G) = (G). We prove that the recognition problem of well-colored graphs is a coNP-complete problem. © 2006 Elsevier B.V. All rights reserved. Keywords: Colorings; Chromatic number; Grundy number; First-fit colorings; NP-complete; Edge dominating sets 1. Why study Grundy numbers? A Grundy k-coloring of a graph G is a proper k-coloring of vertices in G such that each vertex is colored by the smallest integer which has not appeared as a color of any of its neighbors. The Grundy chromatic number (or simply Grundy number) (G) is the largest integer k, for which there exists a Grundy k-coloring for G. As an example, in Fig. 2, a tree with Grundy number equal to 4 is displayed. The Grundy number of a graph was perhaps introduced for the first time by Christen and Selkow [2].In[3], Erdös et al. proved that the Grundy number of a graph is in fact the same as ochromatic number of a graph which was defined and studied independently by Simmons [12].In[9] the authors studied the Grundy number of hypercubes and determined the exact values. From computational point of view, polynomial time algorithms for determining the Grundy number have been given for trees in [8] and for partial k-trees in [13]. In a manuscript [4] the NP-completeness of determining the Grundy number of general graphs has been proved. Therefore, they gave an affirmative answer to the problem 10.4 posed in the graph coloring problem book [10] which asks whether determining the Grundy chromatic number of graphs is an NP-complete problem. The previous paragraph shows the tradition of studies focused on the Grundy number itself, but another importance of Grundy numbers appears when we consider the on-line version of greedy colorings. An on-line coloring algorithm is a coloring algorithm that properly colors a graph G by receiving its vertices in some order v1,v2,...,vn in such E-mail address: [email protected]. 0012-365X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.disc.2005.06.044 M. Zaker / Discrete Mathematics 306 (2006) 3166–3173 3167 a way that the color of vi is assigned by only looking at the subgraph of G induced by {v1,...,vi}. Therefore, the greedy procedure can also be considered as an on-line coloring because at each step in the coloring of a given graph we only need the previous information of the graph. This on-line version is called first-fit coloring denoted by FF and the literature is full of papers on this concept and its own problems, see [5,6,11]. On the other hand since in the definition of Grundy coloring we need the whole information of graph, then one may say that Grundy coloring is an off-line coloring. For each graph G,FF(G) is defined as the maximum number of colors used in a first-fit coloring of G. In fact FF(G) measures the worst case behavior of FF on G and the researches in this area are focused on this worst case measure i.e. FF(G). It can be easily seen that the equality FF(G) = (G) holds for every graph G. On the left side of the equality we have the worst case behavior of an on-line coloring and on the right the value of an off-line chromatic number. Also, in the present paper a close relationship of Grundy colorings with edge dominating sets will be given. 2. Fixed-parameter complexity and t-atoms In this section we introduce the concept of t-atoms, using them we obtain a sufficient and necessary condition to have a Grundy number at least t for any graph G. We discuss the fixed-parameter complexity of determining (G)k, for a fixed integer k. Suppose two graphs G, H and a partition P of the vertex set of H into independent (stable) sets are given. Let P partition V(H)as V(H)= C1 ∪ C2 ∪···∪Ct . We say H is a P-subgraph of G if H can be embedded (as an ordinary subgraph) in G in such a way that each Ci remains a stable subset in G. We define a family of graphs At , t = 1, 2, 3,..., whose elements are called t-atoms. These atoms play the main role in our arguments. At ’s are defined inductively as follows: • A1 ={K1}. • A2 ={K2}. • A typical element G of At can be constructed from an element H ∈ At−1 with |V(H)|=n as follows: Fix 1mn, consider Km ∪ H and a subset W ⊆ V(H)with |W|=m. Construct a perfect matching between W and V(Km) and connect each vertex in V(H)\W to an (and to only one) arbitrary vertex in V(Km). In Fig. 1, construction of an atom is displayed. In the figure we have supposed m = 3. The graph Km and the subset W ⊆ V(H)and a matching between W and Km are displayed. There are only two atoms with t = 3, which are K3 and the path on four vertices P4. It is clear by our construction of t-atoms that the Grundy number of these atoms is at least t. Since a t-atom is of the form Km ∪ G where G is a (t − 1)-atom and the vertices of Km consist of a maximal independent set, hence if (G)t − 1 then (Km ∪ G)t. In fact if V (G) = C1 ∪ C2 ∪···∪Ct−1 is the partition of V (G) which comes from a Grundy (t − 1)-coloring of G (i.e. C1,C2,...,Ct−1 are color classes) then V(Km) ∪ C1 ∪ C2 ∪···∪Ct−1 is a K3 H W Fig. 1. A construction of an atom G by another atom H, where m is taken to be 3. 3168 M. Zaker / Discrete Mathematics 306 (2006) 3166–3173 partition of Km ∪ G which gives rise to a Grundy t-coloring, if we color the vertices of Km by color 1 and the vertices in Ci by color i + 1. For each atom, such a partition is called canonical partition and is obtained uniquely, since for K2 which is the only 2-atom, there is only one such partition. Throughout this paper, by a t-atom we always consider the atom itself together with its canonical partition. Particularly when we say that a graph G contains an atom H we mean the containment with respect to the canonical partition of H . Theorem 1. For a given graph G, (G)t if and only if G contains (with respect to the canonical partition) a t-atom. Proof. First, it is easily seen by the construction of t-atoms that these atoms have a Grundy coloring with t colors. So the Grundy number of a graph which contains such an atom is at least t. Now, consider a graph G with (G)t.We prove by induction on t that G contains a t-atom. Let C1,C2,...,Ct be the color classes of a Grundy coloring of G with t colors, so that Ci,1i t, consists of the vertices colored by i. Let H = G\C1. Obviously (H )t − 1. By induction we conclude that H includes a (t − 1)-atom, say F. Now since C1 is a maximal independent set in G, every vertex of F has a neighbor in C1. Hence it is clear that we can obtain a t-atom, by excluding, if necessary, some vertices in C1 or edges between F and C1, inside G. The structure of atoms is similar to the structure of critical graphs (for the on-line coloring FF), which is introduced in Proposition 1 in [5].In[5] a graph G with (G)=t is said to be a t-critical graph if (G\v)<(G) for all v ∈ V (G). But a t-critical graph is not necessarily a t-atom. Critical graphs will also be used in Section 4. It is noted in [5] that for any positive integer t there are only finitely many t-critical graphs. As an example and result of Theorem 1, we observe that all connected graphs except complete bipartite graphs have Grundy number at least 3, based on this fact that there are only two 3-atoms which are K3 and the path on four vertices P4.